Peripheral Nerve Sheath Tumors: Characteristic Signs on MRI

Abstract

Purposes: To study the characteristic signs and features of peripheral nerve sheath tumors (PNSTs) in patients with or without neurofibromatosis (NF) type-1 or type-2. Materials and methods: Retrospective descriptive study of the MR images of 39 PNSTs in 30 patients which are pathologically proven neurofibromas, schwannomas or malignant peripheral nerve sheath tumors (MPNSTs). Age, sex, affected site, size of tumors and characteristic MRI features were described Results: There are 23 schwannomas and 2 neurofibromas from isolated PNSTs group; 4 neurofibromas and 2 MPNSTs from NF-1group, and 8 schwannomas from NF-2 group. Six (6) neurofibromas are comprised of 3 localized and 3 plexiform subtypes. Fifteen (15) (48.39%) intermuscular and 3 (9.68%) paraspinal schwannomas are of fusiform and dumbbell shapes, respectively. One (1) (33.33%) localized neurofibroma is of a fusiform shape. Entering and existing nerve sign presents in 14 (45.16%) schwannomas and in 1 (33.33%) localized neurofibroma. Six (6) of 14 schwannomas and 1 of 1 localized neurofibroma are eccentrically related to the parent nerves. Nineteen (19) of 20 intermuscular schwannomas and 1 of 1 intermuscular localized neurofibroma demonstrate a split-fat sign. Target sign is found in 11 (35.48%) schwannomas and 3 (100.00%) plexiform neurofibromas. Fascicular sign is seen in 13 (41.93%) schwannomas. All 3 plexiform neurofibromas demonstrate a “bag of worms” appearance. Associated muscular atrophy is seen in 1 (3.22%) schwannomas. Reverse target-pattern enhancement is found in 6 (20.69%) schwannomas. Two (2) MPNSTs are larger than 5 cm, and have indistinct borders and heterogeneous enhancement.

Conclusion: The MR features include a fusiform shape of intermuscular plane lesions, a dumbbell shape of paraspinal lesions, split-fat sign, entering and existing nerve sign, target sign, fascicular sign and reverse target enhancement, which are helpful in diagnosis of schwannomas and localized neurofibromas. However, there is no precise MR feature in differentiating between these two types of tumors. Localized neurofibroma can be eccentrically related to the parent nerve. A “bag of worms” appearance is a pathognomonic sign of the plexiform neurofibromas. Associated muscular atrophy is uncommon in our study. MPNSTs demonstrate larger size, loss of normal fascicular architecture, indistinct borders and more heterogeneity.